Transonic drag rise and centre of pressure shift

The dramatic change in flow from subsonic to supersonic conditions is, as might be expected, accompanied by marked loading changes on the aerofoil. One important consequence of this is a rearward shift in the centre of lift.

The formation of the shock waves as the flow develops in the transonic speed range leads to the formation of a large separated wake (Fig. 5.18(b)). This in turn leads to a very rapid drag rise over a small Mach number range.

This shock wave is a reflection from the tunnel wall
(b)	(c)
Fig. 5.18 Shock wave development on a conventional aerofoil (a) Subsonic flow with no shocks (b) Transonic flow. The approaching flow is subsonic, but patches of supersonic flow develop downstream of the leading edge, terminating in a shock wave on both upper and lower surfaces (c) Supersonic approach flow. Oblique shock waves initiated at the leading edge slow the flow to a lower Mach number than the approach. The flow then accelerates to a higher Mach number, and is finally reduced again via a second pair of shock waves at the trailing edge

The drag rises much more rapidly than the dynamic pressure so that the drag coefficient rises. The drag coefficient falls again as the fully supersonic flow pattern is established and Fig. 5.19 shows the typical transonic drag coefficient peak which is of great importance in the design of both transonic and super­sonic aircraft as we shall see in later chapters.

Figure 5.19 also shows that the lift coefficient varies significantly as the speed of sound is approached. It should be noted that Fig. 5.19 shows the vari­ation of lift and drag coefficients at constant angle of attack. If the angle of attack is varied as the flight speed is changed in order to keep the overall lift (rather than the lift coefficient) constant, as would be the case in cruising flight, then a slight fall in the drag coefficients is frequently experienced just prior to the rapid rise as the speed of sound is approached. This occurs because the increase in lift coefficient means that the angle of attack can be reduced. This local reduction in drag coefficient can be usefully exploited in design.